In the name of Allah.

Salam/Peace,

As for the exact date, careful investigation into circumstantial evidence and relevant clues point directly to Monday, Ramadan 21 at night, that is, August 10, 610 CE, with Prophet Muhammad exactly 40 years, 6 months and 12 days of age, that is, 39 Gregorian years, 3 months and 22 days.
Read more
http//en.wikipedia.org/wiki/Muhammad
http//www.easyislam.com/chronology_of_events_in_the_life_of_muhammad.asp
http//muhammad.islamonline.net/English/Whos_Muhammed/ImportantEvents/01.shtml

Prophet Muhammad a.s. died on 632 CE.
The Prophet a.s. practiced Islamic calendar for 22 years
Read more
http//en.wikipedia.org/wiki/Islamic_calendar
http//www.islamicfinder.org/Hcal/special_days.php

Six (6) years after Prophet Muhammad a.s. passed away, on 638 CE (not too long after prophet died), the Omer kalifa wrote the Islamic Calendar in paper and spread it around the world. What Omer wrote, was what Muhammad a.s. practiced, and what Allah sent to us!

All arguments that we have today, also the Muhammad a.s had on his time! He is last messenger of Allah, he received the Quran on his heart from our GOD, and he explained to us how the shape of the New Moon is and when the Night of Decree is so we don?t harm our selves TODAY!

Quran verses are not to be commented by the people but are to be recorded on the hearts of the humanity. You all who are seeking the truth, learn to recite (put in your heart using your own language so you can understand) verses about the time calculation (2185 2189 936 105 1712 3639 ?) and see the miracle.

Hint
Ask yourself How old am I? Which calendar do I need to use?

4615 We have enjoined on man kindness to his parents In pain did his mother bear him, and in pain did she give him birth. The carrying of the (child) to his weaning is (a period of) thirty months. At length, when he reaches the age of full strength and attains forty years, he says, "O my Lord! Grant me that I may be grateful for Thy favour which Thou has bestowed upon me, and upon both my parents, and that I may work righteousness such as Thou mayest approve; and be gracious to me in my issue. Truly have I turned to Thee and truly do I bow (to Thee) in Islam."

How old was the Prophet Muhammad a.s when he received the First Revelation? Which calendar GOD used in the Quran to count his (and other prophets) years? Which calendar Prophet Muhammad a.s used during his time (22 years) as a prophet?